# Modeling Fluorescence Correlation Spectroscopy through an aberrating sphere

## TL;DR

This paper develops a theoretical model to understand how an aberrating sphere affects fluorescence correlation spectroscopy measurements, crucial for accurate analysis in biological tissues with optical distortions.

## Contribution

It introduces a novel theoretical framework to quantify the impact of aberrating spheres on FCS measurements, enhancing accuracy in complex optical environments.

## Key findings

- Aberrations significantly alter FCS signal properties.
- The model predicts how aberrations influence diffusion and concentration measurements.
- Results aid in correcting FCS data in non-ideal optical conditions.

## Abstract

Fluorescence Correlation Spectroscopy (FCS) is a powerful single-molecule technique which allows for measuring motion (diffusion, flow), concentration, and molecular interaction kinetics of fluorescent molecules from picomolar to micromolar concentrations. It has found manifold applications in the physical and life sciences. Many biological/biophysical applications use FCS for measuring the motion and concentration of fluorescently labeled biomolecules in living cells and tissue. However, a correct quantitative evaluation of FCS experiments relies on the accurate knowledge of the fluorescence excitation and detection properties of the used confocal microscope. Using a bottom-up approach, we theoretically study how these properties are affected by the presence of a diffracting dielectric bead within the optical path, and how this changes the outcome of a FCS measurement. This will be important for all applications of FCS under optically non-ideal aberrating conditions.